I1 78 METALLURGY OF IRON AND STEEL.
; j H
i [ In burning soft coal, no matter whether it be burned directly
in a shallow fire or whether it be first put through a producer and the gas afterward burned in a furnace, the ultimate products of combustion with no excess of air contain C02=18 per cent., N= 82 per cent. The products of combustion from blast-furnace gas are much higher than this in C02 and lower in N, because the ore supplies oxygen without nitrogen, an unusual condition in ordinary processes of combustion. In most operations where fuel is burned, twice the amount of air must be supplied that is theoretically necessary in order to insure the complete burning of all the combustible components in the gas, and the loss of heat arising from this excess is much less than the loss arising from the escape of unburned combustible when the excess of air is too small. Following is the result of burning 100 cubic feet of gas with twice the theoretical quantity of air:
100 cu. ft. gas-j-130.3 cu. ft. air=214.9 cu. ft. products of combustion of the following composition:
C02=17.87 per cent, 0=6.36 per cent., F=75.77 per cent.
The specific heat of gases varies with the temperature. In this case the whole mass of products have a specific heat of .0198 B.t.u. per cu. ft. at a temperature of 32° F., .0213 at 600° F. and .0228 at 1200° F. The specific heat of the excess air contained in these products is somewhat less than the average, being only .0192 at 32° F., but for practical purposes these variations may be ignored, and in calculating the waste of heat in gases escaping at moderate temperatures from boilers or stoves the specific heat may be taken at .022 B.t.u. per cubic foot, and if twice the necessary amount of air has been used so that the excess air constitutes 30 per cent, of all the products of combustion, it may be assumed that this air carries away 30 per cent, of the wasted heat. The gas C has a calorific value of 95.56 B.t.u. per cubic foot, but counting its sensible heat at 500° F. its value is 105.7 B.t.u. The value of 100 cu. ft. will be 10,570 B.t.u., and the heat lost in the products of combustion under different conditions are as shown in Table II-H.
The temperature of gases escaping from boilers ranges from 500° F. with fairly good practice to 1100° F. or more with bad practice. The loss of heat due to this cause is 22 per cent, of the total